Added mechanism to show docstrings of plugin classes as a part of
plugins help page.
For missing plugins the following warning message is shown:
<class '<plugin class spec>'> is missing docstring.
[YOCTO #7118]
(From OE-Core rev: 0997208266686473d23aed0fab58a1fd7c5d8cae)
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>